Carbon fiber reinforced plastic laminates are a well known class of materials that are used in structural applications ranging from rocket motor cases to golf club shafts due to its high stiffness to weight ratio and high strength to weight ratio, which can be much higher than metals. The carbon fiber laminates consist of various carbon fiber plies (laminae), oriented and stacked in a prescribed pattern (analogous to plywood) tailored to meet the structural requirements of the member. Each ply of the laminate consists of carbon fibers embedded in a plastic matrix.
One of the primary advantages of composite structures over metallic structures is that they are more tailorable to design requirements. For example, a pressure vessel has twice the load in the hoop direction than the axial direction. A metallic vessel would be sized in thickness to meet the hoop load but would be oversized for the axial loads because a metal is isotropic, that is, having the same properties in all directions. On the other hand, a composite pressure vessel could be designed with just enough fibers oriented in the hoop direction to react the hoop loads and just enough fibers oriented axially to react the axial loads. Therefore, the composite pressure vessel would be a more efficient structure due to the tailorability of the composite materials.
Optical support structures in imaging systems must possess excellent dimensional stability during testing and operations. In optical support structures, instead of tailoring the structure to react loads like a pressure vessel, the plies are oriented to optimize (control) the coefficient of thermal expansion (CTE) of the structure. In general, the design goal of an optical support structure is zero CTE or near-zero CTE. To achieve a near-zero CTE laminate using one fiber throughout the laminate in two orthogonal directions (2D laminate), a quasi-isotropic layup is typically used. The quasi-isotropic layup produces material properties (modulus, CTE, etc.) that are equal in the 2D plane of the laminate
One limitation to the use of quasi-isotropic laminates composed of one material system is that the laminate can not be tailored for loads or strength as described in the pressure vessel example. A material system is composed of one fiber and one resin. Using two or more material systems, it is possible to tailor the laminate for zero CTE in both directions while retaining the ability to tailor the modulus or strength to gain efficiencies for load and stiffness.
For applications such as optical support structures, it would be desirable to produce a carbon fiber laminate with a specified in-plane CTE that is near zero and equal in all directions in the plane (quasi-isotropic). The fibers currently available will produce two dimensional carbon fiber laminates with only discrete values of CTE and modulus which are not adjustable for the needs of specific applications.